Force spectroscopy on DNA by FM-AFM

We present imaging and force spectroscopy measurements of DNA molecules adsorbed on functionalized mica. By means of Non-Contact mode AFM (NC-AFM) in Ultra High Vacuum (UHV), the frequency shift (\Delta f) versus separation (z) curves were measured providing a quantitative measurement of both force and energy of the tip-DNA interaction. Similarly, topographic images of the adsorbed DNA molecules in constant frequency shift mode were collected. The high resolution force measurements confirm the imaging contrast difference between the substrate and DNA. The force curves measured along the DNA molecule can be divided into two classes showing marked differences in the minimum of the interaction force and energy, indicating that NC-AFM could deliver chemical contrast along the DNA molecule.